Powdered, curable phenol-formaldehyde resins (resole resins) have found wide use as an adhesive for a variety of wood composites, particularly oriented strand board (OSB) and other similar wafer or chip board products. Phenol-formaldehyde resole resins are typically prepared by reacting a molar excess of formaldehyde with phenol under liquid, e.g., aqueous, alkaline reaction conditions. The resulting liquid phenol-formaldehyde resole resin is then spray-dried to produce the curable phenol-formaldehyde resin powder that is used as an adhesive. Methods for preparing curable phenol-formaldehyde resin powders by spray-drying are discussed and described in U.S. Pat. Nos. 4,098,770; 4,424,300; 4,708,967; 4,950,433; 4,945,000; and 6,608,162.
Adjusting or controlling the packed bulk density of the spray-dried powder within an appropriate range has been a common goal of spray-drying operations. Generally, a packed bulk density for the resole resin powder between about 0.2 g/cm3 and about 0.8 g/cm3 is suitable. Oftentimes, lower packed bulk density resin powders exhibit better mill performance in the manufacture of composite wood products as compared to higher packed bulk density powders. As such, it is often desirable to produce lower packed bulk density resin powders.
In the case of spray-drying phenol-formaldehyde resole resins, it is believed that the packed bulk density of the spray-dried resin is the result of a complex inter-play, inter alia, air feed rate and temperature, liquid feed rate and temperature, and/or liquid droplet size and the solids concentration of the feed liquid. Typically, the amount by which the phenol-formaldehyde resole resin is diluted with water and/or other liquid(s) before spray-drying is used to adjust the packed bulk density of the spray-dried resin powder. Provided that the powder is thoroughly dried, a higher level of dilution usually results in a lower packed bulk density for the recovered spray-dried powder. Increasing dilution of the phenol-formaldehyde resole resin, however, increases drying energy consumption and decreases powder output.
There is a need, therefore, for improved methods for adjusting, controlling, or otherwise influencing the packed bulk density of spray-dried phenol-formaldehyde resole resin powders.